M.S.
Hi C.,
What a great question! I don't know where you live in Seattle, but if you are willing to travel to Bellevue, you might want to try Meydenbauer or Clyde beaches. The Blue Angles use Sacred Heart Catholic Church in Clyde Hill as an orientation point for some of their turns and dives (the church has a tall steeple on a hill), so west Bellevue gets some AWESOME flyovers.
Meydenbauer and Clyde Beaches are near downtown Bellevue, so try google or mapquest for directions.
Another eastside beach option is Medina Beach near Medina Elementary school. There's not much parking at the beach(it's near the Medina Police Station) so I'd recommend parking at the school and walking, it's not far to walk to the beach. Medina Store is a few blocks away for snacks and drinks.
Obviously, I-90 is closed for the Blue Angles, but all 3 beaches are easily access by 520.
Enjoy the show!